Many types of magnetic sensor apparatuses and non-contact-type electric current sensor apparatuses utilizing magnetic sensor apparatuses have been long developed since such apparatuses are useful in industry. However, their application fields have been limited and the market scale have been thus limited. Consequently, development of such apparatuses in terms of cost reduction have not been fully achieved yet.
However, emission control originating from the need for solving environmental problems has accelerated development of electric automobiles and solar-electric power generation. Since a direct current of several kilowatts to tens of kilowatts is dealt with in an electric car or solar-electric power generation, a non-contact current sensor apparatus is required for measuring a direct current of tens to hundreds of amperes. The demand for such current sensor apparatuses is extremely high. It is therefore difficult to increase the popularity of electric automobiles and solar-electric power generation unless the current sensor apparatuses not only exhibit excellent properties but also are extremely low-priced. In addition, reliability is required for a period of time as long as 10 years or more for a current sensor apparatus used in a harsh environment as in an electric car. As thus described, it has been requested in society to provide current sensor apparatuses that are inexpensive and have excellent properties and long-term reliability.
For non-contact measurement of an electric current, an alternating current component is easily measured through the use of the principle of a transformer. However, it is impossible to measure a direct current component through this method. Therefore, a method is taken to measure a magnetic field generated by a current through a magnetic sensor for measuring a direct current component. A Hall element is widely used for such a magnetic sensor. A magnetoresistive element and a fluxgate element are used in some applications, too.
For example, the following problems have been found in the current sensor apparatus utilizing a Hall element that has been most highly developed in prior art.
(1) low sensitivity
(2) inconsistent sensitivity
(3) poor thermal characteristic
(4) offset voltage that requires troublesome handling
In addition to the above problems, a magnetoresistive element has a problem of poor linearity.
Some methods have been developed for solving the problems of a Hall element. One of the methods is a so-called negative feedback method, that is, to apply a reversed magnetic field proportional to an output of the element to the element so as to apply negative feedback such that the output of the element is maintained constant. Consistency in sensitivity, the thermal characteristic, and linearity are thereby improved.
When the negative feedback method is used, however, it is required to apply an inverse magnetic field as large as the field to be measured to the element. Consequently, when a current as high as hundreds of amperes is measured in applications such as an electric car or solar-electric power generation, a feedback current obtained is several amperes even if the number of turns of the coil for generating a feedback field is 100. Therefore, a current sensor apparatus embodied through this method is very large-sized and expensive.
If the magnetic sensor element has high sensitivity, it is possible that a feedback current is reduced by applying only part (such as one hundredth) of the field to be measured to the element. However, this is difficult for a Hall element with low sensitivity used as the magnetic sensor element.
As thus described, it is difficult in prior art to apply the negative feedback method to a current sensor apparatus used for non-contact measurement of a large current containing a direct current component. It is therefore difficult to implement an inexpensive current sensor apparatus having excellent characteristics.
A fluxgate element has been developed mainly for measurement of a small magnetic field while not many developments have been made in techniques for measuring a large current. However, with some modification a fluxgate element may be used as a magnetic detection unit of a current sensor apparatus for a large current since the fluxgate element has a simple configuration and high sensitivity.
Reference is now made to FIG. 25 to describe the operation principle of a fluxgate element having the simplest configuration. FIG. 25 is a plot for showing the relationship between an inductance of a coil wound around a magnetic core and a coil current. Since the core has a magnetic saturation property, the effective permeability of the core is reduced and the inductance of the coil is reduced if the coil current increases. Therefore, if bias magnetic field B is applied to the core by a magnet and the like, the magnitude of external magnetic field H.sub.o is measured as a change in inductance of the coil when external field H.sub.o is superposed on the bias field. This is the operation principle of the simplest fluxgate element. In FIG. 25 each of bias field B and external field H.sub.o is expressed in the magnitude converted to the coil current.
In this method the position of bias point B changes with factors such as the intensity of the magnetic field generated by the magnet or the positions of the magnet and the core in relation to each other. It is therefore required to maintain the inductance at a specific value when the external magnetic field is zero. However, it is extremely difficult to compensate the instability of the inductance value due to temperature changes and other external perturbations. This method is therefore not suitable for practical applications.
If a rod-shaped magnetic core is used, an open magnetic circuit is provided, so that the effect of hysteresis is generally very small. Assuming that the hysteresis of the core is negligible, the characteristic of variations in inductance is equal when the coil current flows in the positive direction and in the negative direction since the saturation characteristic of the core is independent of the direction of coil current. For example, in FIG. 25 it is assumed that point P.sub.+ and point P.sub.- represent the coil current in the positive direction and the coil current of the negative direction, respectively, whose absolute values are equal to each other. In the neighborhood of these points, the characteristic of variations in inductance with respect to variations in the absolute value of the coil current is equal. Therefore, an alternating current may be applied to the coil such that the core is driven into a saturation region at a peak, and the difference in the amount of decrease in inductance may be measured when positive and negative peak values of the current are obtained. As a result, the difference thus measured is constantly zero when the external magnetic field is zero, which is always the case even when the characteristics of the core change due to temperature changes or external perturbations. In the present patent application a saturation region of the magnetic core means a region where an absolute value of the magnetic field is greater than the absolute value of the magnetic field when the permeability of the core is maximum.
An external magnetic field is assumed to be applied to the core. If external field H.sub.o is applied in the positive direction of the current, as shown in FIG. 25, the inductance value decreases at the positive peak of the current (point Q.sub.+ in FIG. 25, for example) and the inductance value increases at the negative peak of the current (point Q.sub.- in FIG. 25, for example). Therefore, the difference between the values is other than zero. Since the difference in inductance depends on the external magnetic field, the external field is obtained by measuring the difference in inductance.
The method thus described is called a large amplitude excitation method in the present patent application, that is, to apply an alternating current to the coil such that the core is driven into a saturation region at a peak, and to measure the difference in the amounts of decrease in inductance when positive and negative peak values of the current are obtained.
Magnetic sensor apparatuses that utilize such a large amplitude excitation method are disclosed in Published Examined Japanese Patent Application Sho 62-55111 (1987), Published Examined Japanese Patent Application Sho 63-52712 (1988), and Published Unexamined Japanese Patent Application Hei 9-61506 (1997), for example. In Published Examined Japanese Utility Model Application Hei 7-23751 (1995), a technique is disclosed to achieve measurement similar to the large amplitude excitation method through the use of two bias magnets.
The large amplitude excitation method is an excellent method since the effects of temperature changes and external perturbations are eliminated. However, because of the following problems, for example, it is not so easy to apply an alternating current enough to drive the core into saturation.
First, if the number of turns of the coil is increased, magnetomotive force obtained with the same current value increases, but the inductance of the coil increases. Consequently, it is required to reduce the frequency of the alternating current supplied to the coil or to increase the voltage. However, an increase in voltage is limited by the supply voltage of the apparatus. A reduction in frequency results in a reduction in response frequency limit of the sensor. Therefore, an increase in voltage or a reduction in frequency is not always acceptable.
Next, if the magnetic core is made extremely thin to facilitate saturation, a portion of the coil inductance owing to the core is reduced and a variation in inductance is reduced. That is, the sensitivity of the magnetic sensor apparatus is reduced.
Next, in order to improve the magnetic properties and obtain a sharp saturation characteristic, a specific magnetic core, that is, an expensive core that is not mass-produced is required, and the price of the apparatus is thereby raised. Furthermore, it is not certain whether a magnetic core newly developed for a magnetic sensor apparatus would maintain reliability for a period of time as long as ten years or more in a harsh environment as in an electric car.
As described so far, it is difficult in prior art to provide a magnetic sensor apparatus to satisfy the demands in society as described above.
In pages 135 to 137 of Toyoaki Omori ed. `Fukyu-ban Sensor Gijutsu (Popular Edition of Sensor Technology)`, Fuji Technosystem, published on Jul. 18, 1998, a technique is disclosed to form a resonant circuit made up of a sensor coil for measuring an external magnetic field and a capacitor and to generate an alternating current used for the large amplitude excitation method by an oscillation circuit incorporating the resonant circuit.
In this technique, however, two coils are wound around the single core, that is, an excitation coil for passing an alternating current for the large amplitude excitation method, and the sensor coil. As a result, manufacturing costs are raised.
Examples in which excitation is achieved through the use of a self-excited resonant circuit in a magnetic sensor apparatus using the large amplitude excitation method are disclosed in Published Unexamined Japanese Patent Application Sho 60-57277 (1985), Published Unexamined Japanese Patent Application Hei 4-24574 (1992), Published Unexamined Japanese Patent Application Hei 6-94817 (1994), U.S. Pat. No. 4,384,254, and U.S. Pat. No. 4,626,782.
Examples of output detection methods in a magnetic sensor apparatus using a fluxgate element are disclosed in Published Unexamined Japanese Patent Application Hei 2-287266 (1990), Published Unexamined Japanese Patent Application Hei 3-135780 (1991), Published Unexamined Japanese Patent Application Hei 3-191870 (1991), and U.S. Pat. No. 4,503,395.
In a magnetic sensor apparatus using a fluxgate element, some means are required for detecting a variation in inductance of the sensor coil to detect a magnetic field to be measured when an alternating current used for the large amplitude excitation method is supplied to the sensor coil through the use of a resonant circuit incorporating the sensor coil. That is, to detect a variation in inductance of the sensor coil, a variation in voltage across the sensor coil may be detected, or a variation in resonant frequency may be detected. However, the former method has a problem that it is difficult to detect a variation in voltage in a resonant state. The latter method has a problem that, since the resonant frequency is proportional to the square root of the inductance, the sensitivity is low and not practical.
In the magnetic sensor apparatus using the fluxgate element, when an alternating current used for the large amplitude excitation method is supplied to the sensor coil through the use of the resonant circuit as described above, to further adopt the negative feedback method, it is required to supply a negative feedback current so that resonance will not stop. Therefore, some means are further required.
In Published Unexamined Japanese Patent Application Sho 60-185179 (1985) and Published Unexamined Japanese Patent Application Hei 9-257835 (1997), examples in which the negative feedback method is adopted to a magnetic sensor apparatus using a fluxgate element are disclosed. In these examples, however, a case in which an alternating current used for the large amplitude excitation method is supplied to a sensor coil through the use of a resonant circuit is not considered.
As described above, the large amplitude excitation method is an excellent method since the effects of temperature changes and external perturbations are eliminated. However, even though this method is adopted, the following problems are still found in an actual magnetic sensor apparatus or current sensor apparatus.
In the large amplitude excitation method, according to the principle, when the external magnetic field is zero, the difference in amounts of decrease in inductance of the sensor coil (hereinafter called an amount of the inductance variation) at positive and negative peak values of excitation current is constantly zero, which is supposed to be unchanged even if the characteristics of the sensor core change due to a change in temperature or external perturbations. In fact, however, the positive and negative peak values of excitation current are not completely symmetric due to distortion of the waveform of the excitation current. As a result, even when the external magnetic field is zero, there is a slight difference in inductance values at positive and negative peaks of the excitation current. This difference changes in direct proportion to the magnitudes of variations in inductance at positive and negative peaks of excitation current. Therefore, a measurement error (hereinafter called an offset error) of the sensor apparatus resulting from distortion of the excitation current waveform when the external field is zero is affected by external perturbations that cause variations in inductance of the sensor coil at positive and negative peaks of excitation current.
A case in which the negative feedback method is applied to a sensor apparatus is considered. The sensor apparatus may cause an offset error that is affected by external perturbations. In the negative feedback method, according to the principle, a canceling magnetic field is applied to the sensor core so that the external field applied to the core is cancelled, and the field inside the core is controlled to be constantly zero. The fact that the field inside the core is zero is detected when the difference in the amounts of inductance variations at positive and negative peaks of excitation current is zero. Since the unknown external field is equal to the known canceling field, the external field is obtained from the canceling field.
Assuming that there is an offset error that is affected by external perturbations, the difference in the amounts of inductance variation at positive and negative peaks of excitation current is zero, due to the above-mentioned canceling field. However, the magnetic field inside the sensor core is not zero, but an inverse direction field is applied to the core because of the canceling field. The inverse direction field causes a difference in amounts of inductance variations that corresponds to a measurement signal whose absolute value is equal to the offset error.
Accordingly, the fact that the offset error changes in response to external perturbations is equal to the fact that the field (that corresponds to the measurement signal of the sensor apparatus) applied for canceling the external field is affected by external perturbations. Therefore, when there is asymmetry between positive and negative peak values of excitation current, due to distortion of the excitation current waveform, the measurement signal of the fluxgate magnetic sensor apparatus or current sensor apparatus implemented through the combination of the large amplitude excitation method and the negative feedback method is affected by external perturbations that may cause variations in inductance of the sensor coil at positive and negative peaks of excitation current.
Such external perturbations include:
(1) those resulting from changes in supply voltage of the excitation drive circuit of the sensor coil PA1 (2) those resulting from changes in temperature
(1-1) changes in excitation current amplitude due to changes in supply voltage PA2 (1-2) changes in distortion of excitation current waveform due to changes in supply voltage PA2 (2-1) changes in magnetic properties of the core due to temperature changes PA2 (2-2) sensor coil deformation due to heat PA2 (2-3) changes in stress of the core due to thermal expansion and so on PA2 (2-4) sensor coil heated due to excitation current or feedback current PA2 (2-5) changes in excitation amplitude due to thermal characteristics of the excitation circuit or those of active elements, in particular.
It is extremely difficult to compensate for a number of factors causing inductance variations that result from various sources individually. In prior art, for example, a constant voltage source has been used for the supply source of the sensor apparatus to reduce supply voltage variations, and a temperature sensing element such as a thermistor has been used to compensate for thermal characteristics. However, adopting a constant voltage source causes not only an increase in cost but also an increase in power consumption. If a thermistor is used to compensate for thermal characteristics, it is difficult to determine compensation characteristics, and moreover, compensation is not fully achieved in many cases.
As described so far, it may be possible to implement a magnetic sensor apparatus or current sensor apparatus having excellent stability against external perturbations through adopting the large amplitude excitation method or the negative feedback method to a fluxgate magnetic or current sensor apparatus. However, the feature of those methods, that is, `excellent stability against external perturbations` has not been fully appreciated in prior-art fluxgate magnetic or current sensor apparatuses since variations in inductance of the sensor coil at positive and negative peaks of excitation current are caused by external perturbations and a measurement error may be varied.